The increasing number of road accidents has led to the evolution of vehicular ad hoc networks (VANETs), which allow vehicles and roadside infrastructure to continuously broadcast safety messages, including necessary information to avoid undesired events on the road. To support reliable broadcast of safety messages, distributed time division multiple access (D-TDMA) protocols are proposed for medium access control in VANETs. Existing D-TDMA protocols react to a transmission failure without distinguishing whether the failure comes from a transmission collision or from a poor radio channel condition, resulting in degraded performance. In this article, we present the importance of transmission failure differentiation due to a poor channel or due to a transmission collision for D-TDMA protocols in vehicular networks. We study the effects of such a transmission failure differentiation on the performance of a node when reserving a time slot to access the transmission channel. Furthermore, we propose a method for transmission failure differentiation, employing the concept of deep-learning techniques, for a node to decide whether to release or continue using its acquired time slot. The proposed method is based on the application of a Markov chain model to estimate the channel state when a transmission failure occurs. The Markov model parameters are dynamically updated by each node (i.e., vehicle or roadside unit) based on information included in the safety messages that are periodically received from neighboring nodes. In addition, from the D-TDMA protocol headers of received messages, a node approximately determines the error in estimating the channel state based on the proposed Markov model and then uses this channel estimation error to further improve subsequent channel state estimations. Through mathematical analysis, we show that transmission failure differentiation, or transmission collision detection, helps a node to efficiently reserve a time slot even with a large number of nodes contending for time slots. Furthermore, through extensive simulations in a highway scenario, we demonstrate that the proposed solution significantly improves the performance of D-TDMA protocols by reducing unnecessary contention on the available time slots, thus increasing the number of nodes having unique time slots for successful broadcast of safety messages.

A vehicular ad hoc network (VANET) is an emerging technology which has a great potential in realizing a variety of new applications. This paper presents a new packet routing scheme which allows a vehicle to discover the existence of a gateway to the Internet and to send/receive packets to/from the gateway via multihop communications. The proposed routing scheme is based on a multichannel medium access control protocol, known as VeMAC [1], [2], using time division multiple access. The performance of this cross-layer design is evaluated for a multichannel VANET in terms of the end-to-end packet delay and the percentage of occupied time slots per frame in a highway scenario. Both packet queueing and service delays are considered in the end-to-end delay calculation by modeling each relay vehicle as a queueing system, in which the packets are served in batches of no more than a specified maximum batch-size.

The need of a medium access control (MAC) protocol for an efficient broadcast service is of great importance to support the high-priority safety applications in vehicular ad hoc networks (VANETs). This paper introduces VeMAC, a novel multichannel TDMA MAC protocol proposed specifically for a VANET scenario. The VeMAC supports efficient one-hop and multihop broadcast services on the control channel by using implicit acknowledgments and eliminating the hidden terminal problem. The protocol reduces transmission collisions due to node mobility on the control channel by assigning disjoint sets of time slots to vehicles moving in opposite directions and to road side units. Analysis and simulation results in highway and city scenarios are presented to evaluate the performance of VeMAC and compare it with ADHOC MAC, an existing TDMA MAC protocol for VANETs. It is shown that, due to its ability to decrease the rate of transmission collisions, the VeMAC protocol can provide significantly higher throughput on the control channel than ADHOC MAC.

A vehicular ad hoc network (VANET) is an emerging technology which has a great potential of new applications in safety, traffic optimization, and entertainment. The VeMAC [1], [2] is a medium access control protocol recently proposed for VANETs, which can support efficient broadcast service necessary for high priority safety applications. The VeMAC protocol reserves disjoint sets of time slots to vehicles moving in opposite directions and to road side units (RSUs). The protocol has been evaluated for vehicle-to-vehicle (V2V) communications and under balanced vehicle traffic conditions, in which the densities of vehicles moving in opposite directions on a two-way road are approximately equal [2]. In this paper we investigate the effects of the existence of RSUs and the unbalanced vehicle traffic conditions on the VeMAC performance, via simulations in highway and city scenarios in terms of network throughput and transmission collision rate.